1. Field of the Invention
The present invention relates to a vehicle brake device for an electronic hydraulic brake (EHB) system. More particularly, the present invention relates to a vehicle brake device an electronic hydraulic brake (EHB) system, capable of quickly forming a braking pressure according to pedal force of a driver in an abnormal operation mode of an EHB system, and suitable for generating a greater braking pressure under the same pedal force.
2. Description of the Related Art
In general, if a driver steps on a brake pedal in a normal operation mode, an electronic hydraulic brake (EHB) system detects the displacement of the brake pedal by a displacement sensor to close a cut-off valve, thereby cutting off passages between a master cylinder of a pedal simulator and wheels. Then, an electronic control unit (ECU) calculates a wheel pressure based on a pressure signal transmitted from a pressure sensor to feed-back control a pressure of each wheel by adjusting a pressure of an accumulator, which stores a pressurized liquid generated from a motor pump, using solenoid valves provided in an intake port and an exhaust port of the wheel. When the EHB detects that the driver wants to perform the braking operation, the EHB supplies pedal repulsive force. To this end, as shown in FIG. 1, the EHB system includes a master cylinder 10 and a simulator 40 equipped with a simulation piston 41 and a simulation spring 42 elastically supporting the simulation piston 41 through an opening 13.
In the master cylinder 10 including the simulator 40, if the driver steps on the brake pedal in a normal operation mode, a pressure applied to the brake pedal is delivered to the master cylinder 10 through a first piston 20, and then delivered to a second piston 30 by a first spring 22 supporting the first piston 20. At this time, passages between first and second chambers 21 and 31 and a wheel cylinder (not shown) are blocked by a normally open (NO) valve so that a brake fluid is stagnated, and the first and second pistons 20 and 30 do not move forward. If the driver keeps on stepping on the brake pedal, the first piston 20 moves forward while compressing the first spring 22, and the brake fluid of the first chamber 21 flows into a simulation chamber 43 through a passage between a bore-type step difference part 11 and a sealing element 36. In this case, a pressure is formed in the simulation chamber 43 and the first chamber 21 due to elastic force of the simulation spring 42, so that the driver feels the braking force.
In an abnormal operation mode of the EHB system, since the cut-off valve is in a normally open (NO) state, the brake fluid of the first and second chambers 21 and 31 of the master cylinder 10 flows into the wheel cylinder when the driver steps on the brake pedal. As the brake fluid of the second chamber 31 flows into the wheel cylinder, the second piston 30 moves forward. If the second piston 30 moves forward beyond a fluid passage 12 formed in the bore-type step difference part 11, the passage between the first and second chambers 21 and 31 is blocked by the second piston 30, so that the master cylinder operates similarly to the master cylinder applied to a conventional brake. Accordingly, the braking operation can be achieved by the pressure applied to the brake pedal.
However, according to the braking device of the EHB system, in the abnormal operation mode, it takes a long time to block the fluid introduced into the simulation piston by using the piston which is moved forward in the master cylinder as the pressure is applied to the brake pedal by the driver. That is, it takes a long time to form the braking pressure by the pedal force of the driver, so that, the EHB system cannot quickly form the braking pressure.